One electrophotographic color printer well known in the art is a tandem-type color printer. This printer has four photosensitive drums corresponding to the colors yellow, magenta, cyan, and black and arranged in tandem and parallel to one another.
One such tandem color printer proposed in Japanese unexamined patent application publication No. 2007-72422 has a main casing, and four photosensitive drums that are detachably mounted as a unit in the main casing. This conventional color printer is provided with drum subunits for holding the photosensitive drums, and a pair of side plates disposed on both sides of the drum subunits with respect to the axial direction of the photosensitive drums. A developer cartridge is provided for each of the drum subunits. The developer cartridges each hold a developing roller and are detachably mounted between the side plates. The drum subunits, developer cartridges, and side plates are all detachably mounted in the main casing as a drum unit.